1. Field of the Invention
The present invention relates to an apparatus and method that are used, for example, in a base station of a CDMA (Code Division Multiple Access) communication system to demodulate a CDMA burst signal transmitted burst-wise through random access, and the invention also pertains to a CDMA mobile communication system to which the CDMA demodulating apparatus is applied.
2. Description of the Related Art
Conventionally, a CDMA (Code Division Multiple Access) mobile communication system generally utilizes a random access scheme to establish calls between mobile and base stations. A CDMA receiving device that establishes a call by random access is disclosed, for example, in Japanese Pat. Appln. Laid-Open Gazette No. 178386/98.
FIG. 9 is a block diagram depicting the construction of the conventional CDMA receiving device exemplified in the above-said laid-open gazette. The illustrated CDMA receiving device is used, for instance, in a base station. Upon occurrence of a call, a mobile station sends a CDMA burst signal to the base station by random access. In this instance, the CDMA burst signal contains call-connecting request data and similar communication control data.
The CDMA receiving device in the base station converts the CDMA burst signal, received by an antenna 80, to a base-band type CDMA burst signal in a receiving/demodulating part 81. The receiving/demodulating part 81 provides the CDMA burst signal to a timing detecting part 82. Further, the receiving/demodulating part 81 provides the CDMA signal to a demodulating part 83 as well.
Based on the preamble of the CDMA burst signal provided from the receiving/demodulating part 81, the timing detecting part 82 computes each multipath delay profile. Then, based on the thus obtained delay profile, the timing detecting part 82 detects de-spreading timing for each path. The timing detecting part 82 indicates the thus detected de-spreading timing to each of de-spreading parts 84a, 84b and 84c. 
The de-spreading parts 84a to 84c each de-spread the CDMA burst signal according to the de-spreading timing. As a result, demodulated signals are generated from plural CDMA burst signals sent over the multipath. The thus obtained demodulated signals are provided to a combining part 85. The combining part 85 performs maximal-ratio combining (RAKE combining) of the demodulated signals. In this way, an ultimate demodulated signal is obtained.
However, when assigned the de-spreading timing, the de-spreading parts 84a to 84c de-spread the CDMA (Code Division Multiple Access) burst signal from beginning to end as depicted in FIG. 10. Accordingly, de-spreading of all the CDMA burst signals takes a relatively large amount of time in excess of the entire burst length. That is, a relatively long demodulation time is required. On this account, a transfer delay of an acknowledge signal (ACK signal) that is an answer to the received CDMA burst signal occurs, causing a decrease in the circuit throughput. Hence, there is a fear of incurring a decrease in the channel capacity.
Therefore, an object of the present invention is to provide a CDMA demodulating apparatus and method that permit reduction of the time for demodulating the CDMA burst signal.
Another object of the present invention is to provide a CDMA mobile communication system that enables the channel capacity to be increased through the use of the above-said CDMA demodulating apparatus and method.
To attain the above objective, the present invention is to demodulate a CDMA burst signal sent by random access by: detecting de-spreading timing of the CDMA burst signal in a timing detection time, based on a correlation value between the CDMA burst signal and a reference code; delaying the CDMA burst signal for the timing detection time, and de-spreading, at the detected de-spreading timing, at least that second-half portion of a non-delayed CDMA burst signal which begins after the lapse of the timing detection time to generate a second-half demodulated signal, while at the same time de-spreading, at the detected de-spreading timing, that first-half portion of the delayed CDMA burst signal which extends from its beginning to the beginning of the second-half portion to generate a first-half demodulated signal; and selecting the first-half demodulated signal and the second-half demodulated signal in succession to obtain a demodulated signal corresponding to the whole CDMA burst signal.
According to the present invention, one CDMA burst signal is divided into first-half and second-half portions, which are de-spread in parallel to ultimately obtain a demodulated signal corresponding to the CDMA burst signal in its entirety. Hence, the demodulation time can be reduced as compared with that in the case of de-spreading CDMA burst signal portions one after another. Since this prevents a delay in the transmission of an acknowledge signal (ACK signal) that is an answer to the received CDMA burst signal, the circuit throughput can be increased. Accordingly, the application of the present invention, for example, to a CDMA mobile communication system increases the subscriber capacity of the system.
Further, the present invention is to demodulate a CDMA burst signal sent by random access by: detecting de-spreading timing of the CDMA burst signal, based on a correlation value between the CDMA burst signal and a reference code; delaying the CDMA burst signal for the timing detection time, and de-spreading, based on a first processing clock, at least that second-half portion of a non-delayed CDMA burst signal which begins after the lapse of the timing detection time to generate a second-half demodulated signal, while at the same time de-spreading, based on a second processing clock faster than the first processing clock, that first-half portion of the delayed CDMA burst signal which extends from its beginning to the beginning of the second-half portion to generate a first-half demodulated signal; and selecting the first-half demodulated signal and the second-half demodulated signal in succession to obtain a demodulated signal corresponding to the whole CDMA burst signal.
According to the present invention, the CDMA burst signal is divided into first-half and second-half portions, which are de-spread in parallel to ultimately obtain a demodulated signal corresponding to the CDMA burst signal in its entirety. Hence, the demodulation time can be reduced as compared with that in the case of obtaining a demodulated signal by de-spreading CDMA burst signal portions one after another at a relatively low, fixed speed. Since this prevents a delay in the transmission of an acknowledge signal (ACK signal) that is an answer to the received CDMA burst signal, the circuit throughput can be increased. Accordingly, the application of the present invention, for example, to a CDMA mobile communication system increases the subscriber capacity of the system.